[Roddy]

I need help with this problem.
Calculate the following to prepare a solution:

a) From solid iron (III) sulfate nonahydrate, calculate the grams required to prepare 500.0 mL of 2.117 x 10^-4M Fe(III).

b) The Fe(III) will hydrolyze and precipitate as Fe(OH)₃ unless the acid concentration is greater than 5.2 x 10 ^-3 M H⁺.  A stock solution of 1.0 M HCl is available.  How many mL are required to add to the solution in part a) above?

In part a) I calculated the g of Fe(III) needed as being 0.0297g, but I am not sure if it is correct.  Part b) I don't know where to start. 

[Borek]

0.0297g seems OK.

How many moles of acid will you need in the final solution? Assume volume (500 mL) will not change - you will be able to check it later comparing initial volume with the volume acid added (if the latter is much smaller, it can be ignored).

[Roddy]

What about part B?  That is the part that I really need the help on.

[Borek]

You have either not read the question or my answer.

[Roddy]

I read both, but neither made sense...

What I tried is the dilution formula C₁V₁=C₂V₂
C₁=2.117x10^-4 mol/L
V₁=500.0mL
C₂=5.2x10^-3 mol/L
V₂= unkown

I solved for V₂ and got 20.4mL.  Is that the correct way to set it up?

[sjb]

How many ml of a 1M solution of acid do you need to take to make 500 ml of a 5.2 mM solution?

[Borek]

I read both, but neither made sense...

Seems like you have not understood the question:

C₁=2.117x10^-4 mol/L

Concentration of what?

C₂=5.2x10^-3 mol/L

Concentration of what?

[Roddy]

OK, I think I see what you mean.  I was using the correct formula, just not the correct initial concentration and initial volume is the unknown, correct?

Redoing it I got:
C₁V₁=C₂V₂
C₁=1.0 M
V₁= unkown
C₂=5.2x10^-3 mol/L
V₂= 500 mL

I solved for V₁ and got 2.6 mL.
That seems to be a more reasonable answer.

[Borek]

Now, as I signalled earlier - final volume is 502.6 mL, that means final concentration is not exactly what you wanted. But you are off by less than 2.6/500 - about half %. In the case of final concentration given with two significant digits half % doesn't matter.

[Roddy]

OK, great thanks for your help on this Borek.   I still am working on that second problem I posted in the other thread.  I am not really sure where to go.